The present invention relates to a method of preparing poly(vinyl alcohol) fibers. More particularly, the invention is concerned with a method of preparing high strength and modulus poly(vinyl alcohol) fibers.
Recently, much attention has been paid to development of new high-performance materials, especially, organic polymer materials which are stronger and lighter than metals and ceramics. Among them is the high strength and modulus fiber, which is thought to have high market needs.
So-called Aramid fibers, that is, totally aromatic polyamide fibers, have been industrially produced on the largest scale among the high strength and modulus fibers. However, the Aramid fibers are too expensive to be widely applied and hence development of other high strength and modulus fibers of lower price have strongly been required. Therefore, many attempts have been made to develop such high strength and modulus fibers from high-volume polymers such as polyethylene(PE), polypropylene(PP), polyoxymethylene(POM), and poly(vinyl alcohol)(PVA). Among these non-rigid polymers, PP and POM are relatively low in theoretically attainable modulus because of their spiral chain structure, leading to formation of fibers with low mudulus. On the contrary, PE and PVA are very promising as candidates for high strength and modulus fibers, since they have high theoretically attainable moduli because of their planar zig-zag structure. However, PE fibers may have limited industrial applications because the melting temperature is as low as 130.degree. C., whereas PVA which has the melting temperature as high as 230.degree. C. and is inexpensive in raw material may greatly contribute to industry if high strength and modulus fibers comparable to Aramid fibers can be fabricated from PVA.
Industrially, the PVA fibers have generally been produced by wet spinning from the aqueous solution and widely used in industrial fields. However, the currently produced PVA fibers are quite low in both the strength and the modulus in comparison with Aramid fibers. To enhance the strength and the modulus, organic solutions instead of aqueous solutions have been proposed as the spinning dope. They are (1) glycerine, ethylene glycol, or ethyleneurea solutions from which dry spinning is carried out (Japanese Examined Patent Publication (Tokkyo Kokoku) No. 9768/1962), (2) dimethyl sulfoxide (DMSO) solutions which are wet-spun into organic non-solvents such as methanol, ethanol, benzene, or chloroform (Japanese Unexamined Patent Publication (Tokkyo Kokai) No. 126311/1985), (3) dimethyl sulfoxide solutions from which dry-wet spinning is performed, followed by 20 times drawing of the undrawn fibers (Japanese Unexamined Patent Publication (Tokkyo Kokai) No. 126312/1985), and (4) 2-15 % glycerine or ethylene glycol solutions of PVA with a molecular weight higher than 500,000 which are employed as the dope for gel spinning (U.S. Pat. No. 4,440,711/1984).
However, the fibers obtained by the above methods exhibit in all cases a strength lower than 20 g/d and a modulus lower than 480 g/d, being by far inferior to the Aramid fibers. Thus, no work has hitherto been reported that uses spinning dopes made from a mixture of an organic solvent and water with an appropriate mixing ratio as described in the present invention. As mentioned above, the spinning dopes which have been used for fabrication of high strength and modulus PVA fibers are prepared from a single organic solvent such as glycerine, ethylene glycol, and dimethyl sulfoxide, or from a mixed solvent of an organic solvent and another organic solvent, not water.
The key factor for fabrication of superhigh strength and modulus fibers from non-rigid polymers such as PE, PP, POM, or PVA is how to extend and orient the folded chains along the fiber axis to a very high degree. Through intensive works the researchers of this invention have finally found out that superhigh strength and modulus PVA fibers can be produced by spinning from the dopes of an organic solvent and water mixture having an appropriate mixing ratio.